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Stop Time in Transient Analysis

Hadi Hayati

Hi

I have designed an oscillator which its center frequency is 2GHz and there seems to be a problem with its stop time I guess...

When I choose stop time for example 50ns, the oscillator works well and I can see the output waveforms. And if stop time is greater than 50ns (0.5us for example), then all I have at outputs is a straight line... There is no signal.

Anybody can help me out with this?

 

Regards,

Hadi

 

Parents

  •  Dear Hadi,

     

    I agree with Lawrence. Depending on the value of errpreset you set in your simulation, the default value of the  maximum integration timestep varies with the value of TSTOP. For example, I believe for an errpreset of "moderate", the value of maxstep is TSTOP/50.There are numerous tables in the spectre manual that outline the default value settings for maxstep, reltol, iabstol...for a specific errpreset and TSTOP setting. For an oscillator where the the period is far less than the value of TSTOP, you might manually set the value of maxstep to, say, 1/50 or 1/100 of the expected oscillation period depending on the Q of your resonator.

    You might also check the negative resistance of your sustaining amplifier to assure there is sufficient margin to start-up with your resonator in case the oscillation is not self-sustaining.

    Good luck Hadi,

    Shawn

Reply

  •  Dear Hadi,

     

    I agree with Lawrence. Depending on the value of errpreset you set in your simulation, the default value of the  maximum integration timestep varies with the value of TSTOP. For example, I believe for an errpreset of "moderate", the value of maxstep is TSTOP/50.There are numerous tables in the spectre manual that outline the default value settings for maxstep, reltol, iabstol...for a specific errpreset and TSTOP setting. For an oscillator where the the period is far less than the value of TSTOP, you might manually set the value of maxstep to, say, 1/50 or 1/100 of the expected oscillation period depending on the Q of your resonator.

    You might also check the negative resistance of your sustaining amplifier to assure there is sufficient margin to start-up with your resonator in case the oscillation is not self-sustaining.

    Good luck Hadi,

    Shawn

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